The present invention relates to probe attenuators, and more particularly to a high speed probe attenuator for use especially with video display drivers that provides an idealized response without "dribble-up" effects or overshoot.
For test and measurement instruments probes are used to connect the instrument to a test point on an electronic device being tested. For best results the probes should not introduce any anomalies into an electrical signal as it exists at the test point so that the appearance on the instrument display is a true representation of the signal. With analog signals the bandwidth does not need to be as high as with digital signals. However digital signals have steep transition characteristics between states, so that a much higher bandwidth is required to minimize anomalies introduced by the probe. Further in certain analog applications where very short duration pulses are generated, such as certain optical and high energy physics applications, a much higher bandwidth also is required to capture a true representation of the signal.
Prior probes, such as the P6057 and P6156 low impedance probes manufactured by Tektronix, Inc. of Beaverton, Oreg., United States of America, have specific problems that limit their use for such applications. For example the P6057 is very fragile and breaks easily due to an internal resistor in the probe barrel that is sensitive to pressure and torque. This probe is hard to calibrate and it is impossible to obtain two probes with the same response. The major flaw is the "dribble-up" effect, as shown in FIG. 2, caused by the "skin effect" of a small diameter (&lt;RG174) coaxial cable used for interconnecting the probe to the instrument. This dribble-up effect, due to an extra resistor in the coaxial connector, results in an overshoot at the leading edge of a pulse signal together with a subsequent gradual rise to the final level of the signal. The frequency response of the P6057 is D.C. to 1.4 GHz with approximately eight percent overshoot.
The P6156 is designed to replace the P6057, and is fairly rugged mechanically. However this probe has overpeaking added in its response that adversely affects performance. Although these probes are very repeatable, they still have the dribble-up effect caused by the small diameter coaxial cable. The frequency response is D.C. to 3.5 GHz with nineteen percent overshoot.
A typical circuit configuration for these prior art probe attenuators is shown in FIG. 1. A thick film substrate, typically of ceramic, has a large resistor laid on top coupled in series with a small resistor. An added capacitor is coupled in parallel with the small resistor, and a parallel parasitic capacitor, in the form of a pair of plates below the substrate and above a portion of the large resistor coupled together through a via, is coupled in parallel with the portion of the large resistor.
What is desired is a high speed probe attenuator that has a high frequency response without demonstrating dribble-up or appreciable overshoot.